The present invention pertains to improvements in the field of electrocoagulation printing. More particularly, the invention relates to an improved electrocoagulation printing ink.
In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, Applicant has described a high-speed electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The dispersion containing the olefinic substance and the metal oxide is applied onto the surface of the positive electrode in a manner so as to form on the electrode surface micro-droplets of olefinic substance containing the metal oxide. As described in the aforementioned patent, this may be achieved by means of a device comprising a rotatable brush provided with a plurality of radially extending horsehair bristles having extremities contacting the electrode surface, and a distribution roller arranged in spaced-apart parallel relation to the brush such as to contact the bristles thereof at their extremities The distribution roller has a plurality of peripheral longitudinally extending grooves and is partially immersed in a bath containing the dispersion. As the distribution roller rotates in the dispersion, the grooves are filled with the dispersion which is thus transferred to the bristles to coat the extremities thereof. Rotation of the brush, on the other hand, causes the coated bristles to transfer the dispersion onto the surface of the positive electrode and thereby form the desired micro-droplets of olefinic substance containing the metal oxide. Instead of a brush, use can be made of a roller provided with a plurality of radially extending strips of chamois leather adapted to contact the electrode surface, the strips being coated in the same manner as the bristles. Rotation of such a roller causes the coated strips to impinge upon the surface of the positive electrode such as to transfer thereon the dispersion and thereby form the desired micro-droplets of olefinic substance containing the metal oxide.
The electrocoagulation printing ink which is used to fill the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. The coloring agent can be a dye or a pigment. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is thereafter cleaned by means of a plurality of rotating brushes and a cleaning liquid to remove any residual coagulated colloid adhered to the surface of the positive electrode.
Where the coloring agent used is a pigment, it is necessary to add a dispersing agent for uniformly dispersing the pigment into the ink. When using dispersing agents such as the non-ionic dispersing agent sold by ICI Canada Inc. under the trademark SOLSPERSE 27000, Applicant has observed that the pigment particles settle out after the ink has been left standing for more than 24 hours, resulting in a phase separation requiring mechanical agitation to re-disperse the pigment in the ink. The use of such a dispersing agent has also been found to cause an increase in the viscosity of the ink to above 500 cp, a reduction of the optical density of the coagulated colloid to about 1.20-1.25 as well as formation on the electrocoagulation printed images of an undesirable colored background having an optical density as high as 0.07. In addition, this dispersing agent causes an undesirable gas generation at the negative electrodes.